The invention relates to a general means for reducing intensity fluctuations in lasers.
One of the current, most-challenging problems in physics is the experimental detection of gravitational waves. First predicted by Einstein in his famous 1916 paper on General Relativity, all attempts to the present date to verify the existence of such waves have been inconclusive. Although initial studies involving coincidences between widely-spaced rf gravitational-radiation detectors have not been fruitful, the development of lasers with unusual frequency stability and refined techniques for phase-locking lasers to cavities has led to renewed interest in gravity wave detection by large scale laser-interferometric means. If such experiments are to be successful, extreme low-noise intensity stability will be required of the laser source.
The development of low-noise lasers is also of considerable general experimental, theoretical and commercial interest. The reduction of noise levels is of obvious importance to most experiments in metrology and spectroscopy and not just to the study of gravity wave detectors. In addition, in fiber-optic communications systems noise reduction.
Recently, special attention has been given to the production of light sources with noise levels lower than the quantum limit using methods based upon the production of "squeezed" light states arising in the nonlinear interaction of radiation with matter (E. Giacobino, T. Debuisschert, A. Heidemann, J. Mertz and S. Reynaud, in the Proceedings of NICLOS--Bretton Woods, N.H., Jun. 19-23, 1989 (Academic Press, New York, 1989); p. 180.) and a decrease in noise level below the quantum limit has been reported. (H. J. Kimble, in Atomic Physics II, ed. by S. Haroche, J. C. Gay and G. Gimbera (World Scientific press, 1989); p. 467.)
Also, it is well-known that saturated absorption inside a single laser cavity produces a bistable regime and hysteresis phenomena. (See, for example, V. N. Lisitsyn and V. P. Chebotayev, "Absorption Saturation Effects in a Gas Laser", Zh. Eksp. i. Teor. Fiz. 54, 419 (1968). [Trans. in Sov. Phys. JETP 27, 227 (1968)]; "Hysteresis and `Hard` Excitation in a Gas Laser", JETP Letters 7, 3 (1968).) Positive feedback between the optical field and absorption is evident. Increasing the field leads to a reduction in saturable absorption. Conversely, decreasing the effective losses in the cavity increases the intensity inside the laser resonator. Hence, hysteresis phenomena appear when the change in saturated absorption exceeds the change in saturated gain.
Theoretical analysis has shown that a large increase in quantum fluctuations can occur in a laser with intra-cavity saturable absorption. (A. P. Kazantsev and G. I. Suredutovich, "The Quantum Theory of the Laser", in Progress in Quantum Electronics, ed. by J. H. Sanders and S. Stenholm (Pergamon Press, Oxford, 1975).) It has also been noted that propagation of a strong beam through a nonlinear absorbing medium can exhibit regions of instability in which fluctuations are amplified. (S. L. McCall, "Instabilities in Continuous Wave Light Propagation in Absorbing Media", Phys. Rev. A9, 1515 (1974); also see, B. R. Mollow, "Propagation of Intense Coherent Light Waves in Resonant Media", Phys. Rev. A7, 1319 (1973).)
Further, when the laser beam is transmitted through a Fabry-Perot interferometer filled with absorber, bistability and differential amplification of noise fluctuations can occur. (H. M. Gibbs, S. L. McCall, and T. N. C. Venkatesan, "Differential Gain and Bistability using a Sodium-Filled Fabry-Perot Interferometer", Phys. Rev. Letters 36, 1135 (1976). However, when the incident and output beam from such an absorber-filled cavity are combined interferometrically and the system is operated near the turning points in the bistable region, photon noise can be reduced below the shot noise at nonzero frequency. (S. Reynaud, C. Fabre, E. Giacobino, and A. Heidmann, "Photon Noise Reduction by Passive Optical Bistable Systems", Phys. Rev. A40, 1440 (1989).) This photon noise reduction is associated with a temporal redistribution of the photons inside the cavity. Unfortunately, the optimum squeezing condition in the passive case occurs right at the turning points where the system is on the edge of instability.